The present invention relates to digital communication receivers, and, more particularly, to a novel digital communications receiver for recovering data which has been phase-shift modulated onto a communications system carrier waveform.
It is known that digital information may be transmitted in a communications system by phase-shift modulation, wherein the phase of a carrier waveform is inverted, i.e. shifted by 180.degree., during each bit time interval in which a first binary data value is to be transmitted, and is maintained with the same phase as the immediately preceding bit time interval when the remaining binary data value is to be sent in a particular bit time interval. Alternatively, one phase may represent one binary data value and the 180.degree. shifted phase the second binary data value. It is also known that digital information may be transmitted over powerlines between a central facility and a multiplicity of remote locations tied to the power distribution network. Powerline communication systems of this type are disclosed and claimed in U.S. Pat. Nos. 3,973,087 and 3,973,240, both issued Aug. 3, 1976; U.S. Pat. No. 3,944,723 issued Mar. 16, 1976; U.S. Pat. No. 4,135,181, issued Jan. 16, 1979; and U.S. Pat. No. 4,161,720, issued July 17, 1979, all assigned to the assignee of the present application and all incorporated herein in their entirety for reference. In powerline communication systems, wherein a powerline frequency (typically 60 Hertz (Hz) in the United States) signal is present throughout the entire system, it is known that the signal-to-noise ratio of the received signal can be greatly improved if the transmitted carrier signal frequencies are odd multiples of the first sub-harmonic, i.e. 30 Hz, of that powerline frequency. Methods and apparatus for realizing such improvements in signal-to-noise are described and claimed in U.S. Pat. No. 4,101,834, issued July 18, 1978; and U.S. Pat. No. 4,109,204, issued Aug. 22, 1978, both assigned to the assignee of the present application and incorporated herein by reference in their entirety.
In a data communications system in which the carrier is continuously present, even when the carrier is not modulated to transmit digital data to a receiving location, one presently known receiver for filtering out both systematic pulse and random noise to improve the signal-to-noise ratio is described and claimed in U.S. Pat. No. 3,944,932, issued Mar. 16, 1976, to the assignee of the present invention and incorporated herein by reference in its entirety. The receiver of U.S. Pat. No. 3,944,932 utilizes at least one narrow band-pass commutating filter preceded by at least one carrier-amplitude-limiting stage. The commutating filter requires a large number of precision resistance and capacitance elements having relatively high cost, and also requires careful adjustment of a number of control elements during assembly, further adding to production cost. A receiver having high rejection of undesirable signals, but also having a relatively low production cost, is highly desirable.
In another known receiver, described and claimed in U.S. Pat. No. 4,298,986, issued Nov. 3, 1981, and assigned to the assignee of the present invention and incorporated herein in its entirety for reference, an improved signal-to-noise ratio is achieved, inter alia, by providing a phase-locked loop to generate a loop signal having a frequency essentially locked to the received carrier frequency, whereby synchronous detection is effected.
In some communication systems, there may be insufficient time in the message preamble, i.e. time between first bit of message received and first intelligence data bit of message received, to permit a local oscillator or loop signal to be phase-locked or synchronized to the received carrier fequency prior to the arrival of the first bit of intelligence data which must be correctly decoded, thus making synchronous detection, the optimum technique for reducing the signal-to-noise ratio, impossible.
Where it is not possible to provide synchronous detection for 180.degree. phase shift modulated signals, a differential phase shift detection scheme has been used. In differential phase shift keying (DPSK) detection, each bit of data is compared to a stored replica of the previously transmitted data bit to determine whether the relative phase therebetween is the same or shifted 180.degree.. This bit-by-bit comparison has a number of disadvantages. The limiting signal-to-noise ratio is degraded because the comparison is made against a noisy reference, i.e. the previous bit, rather than against a noise-free reference as is possible with synchronous detection. Further, differential phase shift detection is subject to errors which are more difficult to detect by error detection codes. It is not uncommon, in low signal-to-noise ratio systems using differential decoding, for the received data bit string to become inverted, i.e. transposed 180.degree. from the transmitted signal, and to remain so for the rest of the message. If this reversal occurs at certain key bits, determined by the type of error detection code used, the message may be accepted as correct. An error detection code may have a plurality of these key bits, increasing the chance of an unacceptably high number of erroneous messages being determined to be correct when differential decoding is used.
Most of the noise in the system is from harmonics of the powerline frequency. Thus, the receiver must be capable of maintaining harmonic rejection of these frequencies.
An object of the present invention is to provide a receiver capable of receiving messages in which the message preamble is of insufficient duration to permit a local oscillator signal to be synchronized with the received signal for synchronous detection while maintaining harmonic rejection.
Another object is to provide a receiver wherein the signal-to-noise ratio is increased over a receiver employing a differential phase shift detection technique.